This invention relates in general to dental appliances and in particular to devices used to remove fabricated crowns from teeth restored in such a manner. The fabricated crown, usually of precious metal, porcelain, or a combination, has proven to be a very functional and durable dental restoration, and is intended to be permanently affixed to the underlying tooth. On many occasions, however, it is necessary for the dentist to remove the crown to provide further and more comprehensive treatment, to treat conditions that may arise, or for a variety of reasons. More often than not, it is desirable to remove the crown in such a way that the underlying tooth and its supporting mechanism will remain intact, and will not be harmed or compromised in any way.
A variety of devices have been patented for the purpose of removing crowns from teeth, yet these devices have not received widespread use in the dental profession as the existing devices generally depend to a large extent on the optimum integrity of the underlying tooth structure and the supporting processes. Very often the dentist cannot determine the integrity of the underlying tooth structure and correctly supposes he could damage tooth through use of an existing crown remover. A widespread technique of dentists is, when the integrity of the tooth is in question, to slice the crown in such a manner that it may be easily removed.
Kenney U.S. Pat. No. 989,267 discloses one of the earlier practical devices for the removal of crowns, and proposed removal of the crown without damage to the crown or the natural tooth structure. His invention was comprised of a set of arms that firmly grasped the crown in an unyielding manner and a threaded member that passed through an opening cut in the crown and seated on the underlying natural tooth structure. As the threaded member was turned, a force was applied to the tooth structure, and an equal and opposite force lifted the crown from the tooth. Subsequently Kennedy U.S. Pat. No. 1,041,098, West U.S. Pat. No. 1,109,096, Flagstad U.S. Pat. No. 1,858,081, Wilson U.S. Pat. No. 3,755,901, and Zatkin U.S. Pat. No. 3,889,376 all provide some improvement of the basic mechanism, but each inventor supposes that the underlying tooth is sufficiently strong to resist the forces necessary to dislodge the crown. This method places little stress on the tooth supporting processes, and the likelihood of an inadvertent extraction is remote, but if the underlying tooth structure is weak, if it has been built up through the use of core materials and retention pins, or if it consists of crown cementing medium, the probability of fracturing the core material, dislodging the pins, fracturing the natural tooth material, and causing irreversible damage to the underlying tooth is extremely likely.
Johnson U.S. Pat. No. 1,177,706 addressed this problem by patenting a device which forced inclined members between the crown and the natural tooth at the gingival margin. The use of his device assumed the presence of a 1 millimeter or so horizontal shoulder at the gingival margin--an assumption that is not valid by current concepts of the practice of restorative dentistry.
This same problem was addressed by Curtis U.S. Pat. No. 3,690,007 directed to a device that also embraced in an unyielding manner the crown to be removed and providing an eyelet that the operator could insert a hook or "back action hammer" that would provide a constant or intermittent force in an axial direction away from the crown. The use of such device assumes that the force can be adequately applied in an axial direction (this may prove difficult in the posterior teeth) and that the tooth supporting processes are of adequate strength to resist the force necessary to separate the crown from the tooth. The principle of operation of such device is remarkably similar to devices patented by Maranda U.S. Pat. No. 1,666,860 and Lococo U.S. Pat. No. 4,230,454 for the extraction of teeth in that it has the potential of applying enough force to cause inadvertent extraction of the underlying tooth.
All of the above listed crown removers provide a mechanical means and in most cases a mechanical advantage for removing crowns from teeth. All the devices operate by applying a force to the underlying tooth structure or in the case of Curtis U.S. Pat. No. 3,690,007 applying a force to the tooth support processes. All these methods of operation assume the underlying structures can safely tolerate these forces, and in the case of Johnson U.S. Pat. No. 1,177,706 the underlying tooth was prepared in a specific manner.
A prior art publication which teaches use of a prying action to spread the crown apart and expand the crown circumferentially is Hoffman U.S. Pat. No. 4,196,520. However, this device is not seated on the crown in such a manner that would ensure firm and proper engagement of the crown expanding means, nor does it provide a force multiplying means.